Oil is used as a lubricant and coolant for the components in an axle or axle tube. This coolant prevents over-heating and operates to increase the life of the components within the axle. More oil provides a larger volume to remove heat, however, it is desirable to maintain a low level of oil in rotating axle components to reduce windage loss.
A separate sump volume for holding a larger volume of oil adds cost to the vehicle and potentially requires additional pumps to move the oil to this alternate location. The additional sump would also use space on the vehicle either making service more difficult to other components, or increasing the overall size of the vehicle.